Using inbred mice, we have developed an in vivo system which permits the study of the seeding, differentiation and proliferation of hemopoietic stem cells without prior irradiation of the receipient. These parameters, as well as further definition of the regulation of hemopoiesis in ectopic marrow grafts and the relationship of the hemopoietic inductive environment (H.I.M.) to marrow regeneration are among our current studies and future objectives. Additionally, using a modified Coulter counter and ectacytometer, we are studying red cell deformability and a better definition of function of cell size as measured by the Coulter principle. BIBLIOGRAPHIC REFERENCES: Koerper MA, Mentzer WC, Brecher G & Dallman PR: Developmental change in red blood cell volume: Implication in screening infants and children for iron deficiency and thalassemia trait. The J. of Peds 89:580-583, 1976. Brecher G, Tjio JH, Smith WW & Haley JE: Marrow Regeneration after Mechanical Depletion. Blood 48:679-686, 1976.